The invention relates to a lens-less projection optical system of a reflection type which is used as a display of a projection type, and especially to an improvement of a lens-less projection optical system of a reflection type.
Projectors which are recently used as displays of a projection type are classified into a rear projector and a front projector. In the rear projector, it is the general trend that a picture is projected on a screen by means of a LCD (a liquid crystal display) projector instead of a CRT (a cathode ray tube) projector which has been conventionally used for the same purpose. The LCD projector is required to be thin and lightweight because of its property. In the front projector, it is important to widen the angle of view of the projected picture so that the projector can be used in the ordinary home in a condition that the interval between the screen and the projector is narrow.
In order to meet the aforementioned request, the lens-less projection optical system of the reflection type to be used as the projector has been devised. FIG. 1 shows a typical example of the conventional lens-less projection optical system of the reflection type.
The lens-less projection optical system of the reflection type shown FIG. 1 is composed of three reflecting mirrors 103a, 103b, 103c, each having a reflecting surface shaped into an aspherical surface. A light flux starting from a picture-forming device of a transmission type 102 is successively reflected by the reflecting mirrors 103a, 103b, 103c, and projected on the screen (not shown).
The first reflecting surface 103a is so situated that its concave surface shaped into the aspherical surface is directed to an picture formation surface of the picture-forming device of the transmission type 102. The second reflecting mirror 103b is so situated that its convex surface shaped into the aspherical surface is directed to a light flux reflected from the first reflecting mirror 103a. The third reflecting mirror 103c is so situated that its convex mirror shaped into the aspherical surface is directed to the light flux reflected from the second reflecting mirror 103b. 
The light flux starting form the light source 101 is transmitted through the picture-forming device of the transmission type 102, and successively reflected by the first to third reflecting mirrors 103a, 103b, 103c. The light flux reflected by the third reflecting mirror 103c is projected on the screen.
In this way, the chromatic aberration can be suppressed by using the lens-less projection optical system of the reflection type. Moreover, since the light path can be folded, the optical system can be made compact. Furthermore, since the internal reflection is little, the high contrast and the high resolution of the projected picture can be obtained by the simple structure. As mentioned in the above, the lens-less projection optical system of the reflection type has the various advantages.
However, several disadvantages have been pointed out on the lens-less projection optical system of the reflection type as mentioned later.
The first disadvantage is that it is difficult to increase the angle of view of the projected picture. For example, when the maximum angle of view is made more than 120xc2x0, it becomes necessary to increase the intervals between the adjacent reflecting mirrors 103a, 103b, 103c. Moreover, since the optical paths are widened as the angle of view is widened, the light fluxes distribute on the perspective reflecting mirrors lO3a, 103b, 103c in wide ranges, and it becomes necessary to increase the sizes of the respective reflecting mirrors 103a, 103b, 103c. Especially, the third reflecting mirror 103c is magnified sharply. Since the whole optical system is magnified as the angle of view is widened, it is difficult to widen the angle of view especially in case that the size of the whole optical system is limited. Moreover, since a fan-shaped distortion aberration that a width of a lower portion of the projected picture is narrowed and the higher portion of the same is widened is caused as the angle of view increases, it is also difficult to increase the angle of view from this view point.
The second disadvantage is that the lens-less projection optical system of the reflection type is magnified. Although this disadvantage relates to the aforementioned difficulty in widening the angle of view, there are still the other factors which cause similar problems beside that. Explaining concretely, the optical system is magnified unavoidably in order to correct the distortion aberration. That is to say, since the aforementioned conventional lens-less projection optical system is composed of the three reflecting mirrors 103a, 103b, 103c, the degree of freedom in correcting the distortion aberration is limited. As a result, it becomes necessary to increase the intervals between the adjacent reflecting mirrors 103a, 103b, 103c, and thereby the lens-less projection optical system of the reflection type is magnified.
Accordingly, it is an object of the invention to eliminate the aforementioned disadvantages of the conventional optical system, to prevent the lens-less projection optical system of the reflection type from being magnified, and to provide a lens-less projection optical system of the reflection type which can widen the angle of view of the projected picture.
According to the feature of the invention, a lens-less projection optical system of a reflection type which successively reflects a light flux starting from a picture-forming device by means of four reflecting mirrors, comprises:
a first reflecting mirror which is so situated that its reflecting surface shaped into a concavely curved surface receives the light flux starting from the picture-forming device,
a second reflecting mirror which is so situated that its reflecting surface shaped into a convexely curved surface is directed to the light flux reflected from the first reflecting mirror,
a third reflecting mirror which is so situated that its reflecting surface shaped into a convexly curved surface is directed to the light flux reflected from the second reflecting mirror, and
a fourth reflecting mirror which is so situated that its reflecting surface shaped into a convexly curved surface is directed to the light flux reflected from the third reflecting mirror.
According to the invention written in claim 1, the distances between the adjacent reflecting mirrors can be narrowed, and the sizes of the reflecting mirrors can be reduced.
In the invention written in claim 2, a reflecting surface of at least one of the four reflecting mirrors is shaped into a freely curved surface.
According to the invention written in claim 2, since the reflecting surface of at least one of the reflecting mirrors can be shaped into an ideally curved surface by introducing the freely curved surface, the degree of freedom in correcting the distortion aberration of the light flux reflected from the freely curved surface can be heightened. According to the aforementioned process, the distortion aberration can be corrected.
In the invention written in claim 3,
a reflecting mirror of at leas one of the four reflection mirror is shaped into a freely curved surface, and
a reflecting surface of at least one of the remaining reflecting mirrors is shaped into an asphrical surface.
According to the invention written in claim 3, since the reflecting surface of at least one of the reflecting mirrors can be shaped into the ideally curved surface by introducing the freely curved surface, the degree of freedom in correcting the distortion aberration of the light flux reflected from the freely curved surface. Moreover, the processing cost of the optical system can be reduced by shaping the reflecting surface of at least one of the remaining reflecting mirrors into the aspherical surface which is easy to be processed.
In the invention written in claim 4, the reflecting surface of the fourth reflecting mirror is shaped in to an aspherical surface.
According to the invention written in claim 4, since the reflecting surface of the fourth reflecting mirror having a lager external shape than those of the other reflecting mirrors can be easily processed, the fabricating cost of the optical system can be further reduced.
In the invention written in claim 5, each of the reflecting mirrors is formed of resin.
According to the invention written in claim 5, since the reflecting mirrors can be formed without the grinding and polishing processes which are indispensable in case that the reflecting mirrors are formed of glass, the fabricating cost of the optical system can be sharply reduced.